The display language is only Japanese on NicoNico. You cannot change the language to any language other than Japanese.
If you want to view in other languages, please use the translate function of your browser.
 

▼About the language switch button at the bottom of the page

By using the language switch button at the bottom of the page, you can use the following functions in the video watching page.
‐ Show comments posted in English or Chinese
This is a function that allows users who use the language switch button to use the comment field dedicated to each language.
‐ Translation of video titles and descriptions
This is a function that allows users to translate video titles and descriptions. If the video you are watching has a translated video title and video description, it will be displayed in the language you selected with the language switch button.